How do I prompt the user if a textbox is enabled but empty?

For Each o As Control In Me.Controls
If TypeOf o Is TextBox AndAlso o.Enabled AndAlso o.Text.Trim().Length = 0 Then
MsgBox(String.Format("{0} textbox is empty.", o.Name), MsgBoxStyle.Information)
End If
Next

The thing to do would be to not enable the Submit button until all required fields have been populated. As reeper suggests, use an ErrorProvider set an error message on all empty fileds to begin with and disable the Submit button. As each field is validated only enable the Submit button if all required fields have been set.

The reason? If you loop through the controls in the submit button, then you can add new fields to this form in the future without having to change or add any code. If you have Submit disabled until all fields have a falue, adding a new field to the form requires you to change all of the other fields code to include the new field, as well as copying that code into the field you just added. It's not a hard update but you still have to do it... and forgetting to do so will cause a logic error in your program that you might not find for sometime causing the risk of invalid data being written to the database.That's completely untrue if you've written your code correctly. If you were to validate your controls when you press submit then you have to go through each control. If you add a new control then you have to add it to the list that requires checking.

If you were to disable the the Submit button then you still only have to add any new controls in one place because you do all the checking in one method and you call that method from the validated event of each control. No code written for those existing controls has to change if and when you add new controls.

Good design is good design whether a project is for school or commercial sale. You wouldn't distribute an inferior interface in commercial software compare to a school project. Well written code is well written code and poorly written code isn't.

Private Sub Form1_Load(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MyBase.Load
Me.Button1.Enabled = False
End Sub

Private Sub TextBox1_Leave(ByVal sender As Object, ByVal e As System.EventArgs) Handles TextBox1.Leave
Me.SetButton1State()
End Sub

Private Sub TextBox2_Leave(ByVal sender As Object, ByVal e As System.EventArgs) Handles TextBox2.Leave
Me.SetButton1State()
End Sub

Private Sub SetButton1State()
Me.Button1.Enabled = Me.TextBox1.Text.Trim() <> String.Empty AndAlso _
Me.TextBox2.Text.Trim() <> String.Empty
End Sub